Detecting particles and determining the size thereof plays a central role in many technical problems. For example, particles in a fluid can be detected before and after a filter and the size thereof can be determined, to determine an effectiveness of the filter.
In the case of laser-based detection of particles, a laser beam is coupled into the fluid, for example, a liquid, but typically a gas or gas mixture such as air, which can have the particles to be detected. The laser light can be scattered on the particles and is collected at specific angles. A signal thus arising can be measured and the particle size can be concluded therefrom, for example.
The detectable particle size can be limited by the signal-to-noise ratio (SNR). In the case of large structures, complex optical systems can be implemented to improve the signal-to-noise ratio. In applications in which the most extensive possible miniaturization is advantageous, alternative measures are taken. For example, mirrors or large-area lenses can be used to collect a larger quantity of scattered laser light.
A laser-based particle counter is described in U.S. Pat. No. 5,085,500 A, which absorbs light which is scattered by particles, which traverse a measurement volume in a fluid. To absorb the light, multiple photodetectors are arranged so that only scattered laser beams can be incident thereon. A laser beam, which is generated by a source, passes through multiple apertures to generate a laser beam having suitable properties.